Projection optical systems using a refractive optical system with refractive optical elements made of optical glass for projection display devices and projection televisions have been widely known.
However, when refractive optical systems are used as the projection optical systems, chromatic aberrations occur because of the characteristics of optical glass varying with wavelength. Particularly, the axial chromatic aberration in long focal length optical systems, known as telescope-type optical systems, and the lateral color aberration in short focus optical systems is a major problem.
An optical system formed as a single mirror has been proposed to eliminate such chromatic aberrations. Also, optical systems with a small number of mirrors, more specifically—three mirrors having curved aspheric surfaces, have been proposed in Japanese Laid-Open Patent Applications H10-111458, 2001-222063, 2001-235681, 2003-344772, and Japanese Patent No. 3043583. However, because at least three aspheric mirrors are used in the projection optical systems described in the above-mentioned Japanese laid-open patent applications and Japanese patent, high accuracy of assembly is required and manufacturing costs become high.
Recently, demand for compactness of such projection optical systems has become strong, and a projection optical system with oblique projection favorable to making such devices compact, as shown in Japanese Laid-Open Patent Application 2004-226997, has attracted attention.
Such a projection optical system with oblique projection easily results in deterioration of imaging performance even if only a small deviation from design values occurs, particularly, a big trapezoidal distortion easily occurs due to tilting of the optical elements. Therefore, methods of using plural aspheric mirrors have been proposed to compensate for these drawbacks.
As described above, when plural aspheric mirrors are used in the projection optical systems with oblique projection, high accuracy of assembly is required and manufacturing costs become high.
Accordingly, the development of a projection optical system capable of relaxing the requirements of accuracy of assembly of the optical system while maintaining good optical performance is an urgent necessity for satisfying the requirements for a compact device.